The collection and removal of unwanted material, such as noxious fumes, dust and the like, can be obtained by the use of ducting having a collecting valve capable of being positioned adjacent to a source of unwanted material, the ducting connected to an abstraction system. Some of the uses of such removal devices include welding operations, grinding machines, laboratory operations which can produce fumes from chemical reactions, and surgical operations. Such devices typically comprise a duct assembly having a collecting mouth or inlet at one end, and a connection to an abstraction fan at the other. The assembly has some flexibility so that the collecting mouth or inlet can be stationed at any desired position. The assembly usually needs to be periodically repositioned and some form of support arrangement is required to retain the assembly in any given orientation and to permit easy movement and repositioning. Once repositioned the assembly should retain the new orientation and not be subject to undesirable movement. The mouth or inlet is normally movable in any direction in a plane and also movable in and out of such plane.
Various arrangements exist providing an adjustable assembly, including the provision of internal or external jointed frames which support the duct. Various devices such as lifting or position-balancing means, such as springs and other devices can be used, together with frictional means at joints.
Canadian Patent number 973,012 (Nederman) discloses a folding duct assembly having an internal duct support extending substantially the length of the duct. The duct support comprises a jointed arm having spring counterbalances to counter the weight of the duct. The joints of the duct support incorporate friction pads to retain the duct in a desired position. Jointed duct support structures are also shown in U.S. Pat. No. 5,527,217 (Engstrom) and U.S. Pat. No. 4,860,644 (Kohl et al). In the Kohl et al device, the joints incorporate friction pads to restrict their mobility, while the joints in Engstrom are linked to a retainer wire that in turn is held in place by a friction fit.
It is desirable, and in most cases essential, that there is provided means for holding the duct assembly in position during use, while still permitting easy movement to a new position. These conflicting requirements are difficult to satisfy with friction type joints. Wear at the joints will interfere with acceptable operation and regular adjustment will be required. If the support structure or frame is housed within the ducts of the assembly, such adjustment becomes very difficult. Also, the action of the friction joints can be adversely affected by the material being extracted through the ducts.
In a further aspect, the typical dust or contaminant removal apparatus includes a filter arrangement, whereby the contaminated air is purified before discharge from the apparatus. A typical filter comprises a hollow cylindrical structure having a folded paper filter element. This type of filter is characterized by inconsistent spacing between the filter pleats, resulting in difficulty in removing dust and dirt lodged within the filter. This reduces the life-span and efficiency of the filter. Accordingly, the use of a planar filter element is desirable. Further, it is desirable to provide a means whereby the filter may be automatically cleaned, without removal of the filter from the apparatus. A typical filter-cleaning arrangement is disclosed within U.S. Pat. No. 5,322,535 (Simms), relating to an apparatus for cleaning a hollow cylindrical air filter. The arrangement comprises a travelling nozzle, which both rotates and travels axially relative to the filter, thereby backwashing the filter with a narrowly directed stream of air moving in a countervailing direction to the normal direction of flow. This arrangement is limited for use with a hollow cylindrical filter, and is not capable of use with a planar filter arrangement. In general, it is desirable to provide a moving nozzle arrangement for directing a narrow air stream against the filter for backwash cleaning, rather than an air stream which covers the entire filter simultaneously, as a more efficient backwash cleaning means.